It is common knowledge that tumors move during and in-between delivery of radiation therapy treatments (Webb, 2006a, Langen and Jones, 2001). The reported real-time motion compensation methods are mainly tracking-based. That is, compensation is done effectively by putting the same intensity of radiation beam on the same position in the tumor reference frame at the same time as what was planned. These methods are implemented through linac tracking (Nuyttens et al., 2006, Murphy, 2004), MLC tracking (Keall et al., 2001, Keall et al., 2006, Neicu et al., 2003) or couch tracking (D'Souza et al., 2005), and can be characterized as hardware solutions.
Papiez et al. (Papiez et al., 2005, Papiez and Rangaraj, 2005, Papiez et al., 2007, Papiez et al., 1999, Papiez and Timmerman, 2008, Papiez et al., 1994, Papiez and Langer, 2006, Papiez et al., 2002, Papieza, 2004), McMahon et al. (McMahon et al., 2007a, McMahon et al., 2007b) and Webb et al. (Webb and Binnie, 2006, Webb, 2006b) incorporated the tumor motion into the dynamic MLC leaf velocity optimization. These methods are considered software approaches to motion compensation.
Tracking-based methods intend to fully and instantly compensate motion errors once motion is detected. Such schemes are considered open-loop methods because they do not explicitly model the compensation errors from hardware limitations and/or prediction. These open-loop tracking methods put great demands on hardware such as the velocity and position accuracy of the MLC, linac or the couch etc., as well as on the accuracy of motion prediction.